Expose standard ImGui runtime variant

[antono3]

Summary

• expose upstream_variant = standard and false docking/viewports capability constants
• provide the same public configuration API as the docking branch
• make unsupported configuration return false and dockspace/platform rendering helpers safe no-ops
• verify that standard helpers never reference docking-only symbols

Verification

• ./scripts/check_upstream_variant.sh
• compiled the V GLFW/Vulkan demo against the standard module with the CI-pinned V compiler
• git diff --check